The present invention relates to a liquid crystal display apparatus, wherein an electric field is supplied in a direction parallel to the surface of the substrate, and more particularly to an active matrix type liquid crystal display apparatus which provides a wide viewing angle compatible with a high image quality.
In a conventional liquid crystal display apparatus, an electrode for driving a liquid crystal is formed on the surface of each of two substrates, respectively, so that the electrodes are facing each other across the liquid crystal. The above conventional liquid crystal display apparatus uses a method which is represented by a twisted nematic display mode, wherein the liquid crystal is driven by supplying an electric field in a vertical direction with respect to the two substrates. In such case, transparent electrodes, such as ITO (Indium Tin Oxide), are used.
On the other hand, another mode of operation, wherein the liquid crystal is driven by supplying an electric field in a direction approximately parallel to the substrate using comb shaped electrodes provided on one substrate, is disclosed in JP-B-63-21907 (1988), and U.S. Pat. No. 4,345,249. In this case, the electrodes need not necessarily be transparent, and opaque metallic electrodes having a high electric conductivity can be used.
However, in the display mode wherein the electric field is supplied to the liquid crystal in a direction approximately parallel to the substrate using active elements (hereinafter called an in-plane switching mode), any method to supply an adequate electric field to the liquid crystal without causing interference between the electric fields supplied in a direction parallel to the substrate in a condition where a large amount of wiring exists, and any means to concurrently improve the image quality, have not been disclosed entirely.
Generally speaking, in the in-plane switching mode, the electric fields interfere with each other, because a large amount of wiring is formed on only one of the substrates through which electric signals are transmitted. Accordingly, the electric field supplied to the liquid crystal is influenced by unnecessary electric fields, and so an adequate electric field can not be supplied to the liquid crystal.
Furthermore, an unnecessary electric capacitance is formed between the electrodes, which sometimes causes the voltage supplied to the liquid crystal to fluctuate. The above described phenomena will cause deterioration of the image quality of the liquid crystal display apparatus. Especially, an electric field generated by an image signal electrode for transmitting signals to respective pixels having an active element, such as a TFT and the like, influences the electric field between a pixel electrode for operating the liquid crystal and the common electrode.
The potential of the image signal electrode varies always during the frame period in the course of transmitting signals. It has been known that, if the pixel electrode (its potential is in a floating condition when the active element is in an off condition) is located close to the image signal electrode, a nonuniformity appearing like shadow stripes referred to as a smear, similar to a cross talking phenomenon, is generated in parallel with the image signal electrode depending on the varying potential of the image signal electrode. In order to suppress this phenomenon, a technique to arrange the common electrode, which is always supplied with a potential from an external source, as the closest electrode to the image signal electrode has been developed by the present inventor (U.S. patent application Ser. No. 08/374,531).
However, in accordance with the above technique, the shielding effect for the electric field is not necessarily sufficient, and the problem caused by generation of the smear phenomenon still exists. Although the smear phenomenon could be suppressed by increasing the shielding effect with broadening of the width of the common electrode, the broadening of the width of the common electrode causes another problem in which the aperture ratio of the liquid crystal display apparatus is decreased.